1. Field of the Invention
This invention relates to digital processing circuitry and more particularly to low power circuits for digital processing.
2. Prior Art
Electronic calculator systems of the type having all the main electronic functions within a single, large scaled integrated (LSI) semiconductor chip or small numbers of chips are described in the following prior applications or patents assigned to Texas Instruments Incorporated:
U.S. Pat. No. 3,819,921 by Kilby et al for "Miniature electronic Calculator", based on an application originally filed Sept. 29, 1967;
U.S. Pat. No. 4,074,351 by Boone and Cochran for "Variable Function Program Calculator";
U.S. Pat. No. 3,819,957 by Bryant for "Digital Mask Logic in Electronic Calculator Chip"; and
U.S. Pat. No. 3,987,416 by Vandierendonck Fisher and Hartsell for "Electronic Calculator With Display and Keyboard Scanning".
These prior inventions made possible vast reductions in cost and size and increases in functions of electronic calculators. Many millions of such calculators have been produced. The efforts to reduce manufacturing costs and increase the functions available to the user are continuing. Particularly, it is desired to provide a basic chip structure that is quite versatile and can be used for many different types of calculators and similar digital processing equipment. This permits a single manufacturing facility to produce a large quantity of the same devices, differing only in a single mask change, and to produce a dozen variations while still maintaining large volume cost advantages.
The previous MOS/LSI calculator chips as referred to above were generally register organized in that a single instruction word operated on all of the digits in a given register. A more versatile approach is to make the machine digit organized, operating on one digit at a time. For example, it may be desired to test or set a particular one bit flag. In a register machine, an entire 13 digit register must be addressed and mask to implement this, whereas a digit organized machine may access only the needed digit or bit. An example of such a processing chip is disclosed in U.S. Pat. No. 3,991,305 by Caudel et al entitled, "Electronic Calculator or Digital Processor Chip with Multiple Code Combinations of Display and Keyboard Scan Outputs". This patent discloses what is commonly known in industry as the TMS 1000 architecture for a 4 bit microcomputer. Another approach using this same type of architecture is disclosed in U.S. Patent application Ser. No. 216,113 entitled, "Dual Register Digital Processor System" by Koeppen, Rogers, Solimeno and Brown. The architecture of disclosed herein is similar to these TMS 1000 architecture and the architecture disclosed in the above applications implemented with low power circuitry.
FIG. 1A illustrates the prior art attempt at low power operation using positive channel MOS field effect transistor devices. This type of circuit is referred to as precharge and conditional discharge circuitry. The node 800 becomes charged during Phi 3. It should be noted that since the circuitry is presented in P-MOS, the devices are active during the negative portions of the timing signals. This node remains charged until conditionally discharged by the input line during Phi 1. If the input line remains high, then the node will remain charged and the output will remain a -V as shown in FIG. 1B. However, if the input is low thus activating device 801, the node 800 will be discharged during phi 1 as shown. The disadvantage to this standard precharge discharge logic is that the precharge period can cause problems in other circuits, such as in addressing RAM cells. If the precharge discharge is connected directly in the addressing portion of the RAM cell, all the addresses are ON during the precharge time. Therefore, if precharge discharge logic is to be used to address a RAM, additional circuitry is required to buffer the precharge intervals from the addressing lines of the RAM cells.
FIG. 2 illustrates a static inverter which includes a device with the depleted region 802 to provide charge at the node connected to the output line. The static inverter removes the precharge problem, however, the static inverter also consumes a larger amount of d.c. current. A static inverter also requires that the size of the load device to be much larger than any of the devices in the precharge discharge circuitry. This is a disadvantage when fabricating the circuitry on a small silicon chip.
A third approach to the low power circuit operation is shown in FIG. 3, which is a complementary MOS inverter. The clocked CMOS inverter does not have precharges and does not require constant d.c. current. However, the CMOS fabrication process is more expensive and more complex than a normal PMOS or NMOS fabrication process.
The low power approach to many semiconductor display applications has included the use of CMOS, precharge/discharge and static devices. Once such application is curcuitry required for liquid crystal displays. Liquid crystal displays require low amounts of power and thus interface well with low power processing circuitry. A reference for liquid crystal display requirements is the International Handbook of Liquid Crystal Displays 1975-76, Second Edition, with 1976 Supplement by Martin Tobias, published by Ovum Ltd. 14 Pen Road, London, NC 9RD, England. Another reference is "General Information on Liquid Crystal Display", published by Epson America, Incorporated, 2990 West Lomita Boulevard, Tolerance, Californai. A third reference is an article entitled, "Liquid Crystal Displays" by L. A. Goodman, printed in the Journal of Vacuum Science and Technology, Vol. 10, No. 5, September/October 1973.
In the past, the LCD devices have required the use of low power circuitry such as the precharge discharge logic, or CMOS logic. This specification discloses another alternative, low power circuit that makes possible a low power interface to LCD's without the disadvantages of the two prior art circuits.
This specification also discloses a low voltage RAM cell. RAM cells are included in the prior named patents. However, this specification describes a technique to fabricate a low voltage RAM cell.
Other patents including similar techniques are U.S. Pat. No. 4,061,506 entitled "Correcting Doping Deffects" by McElroy and U.S. Pat. No. 4,280,271 entitled "Three Level Interconnect Process for Manufacture of Integrated Circuit Devices" by Lou, Ponder and Tubbs.
In past calculators and microcomputer chips, low power CMOS circuitry or static logic have been used to fabricate oscillators in clock circuitry. This specification discloses a technique to fabricate low power oscillator circuitry without the disadvantage of precharge discharge circuitry, static converters and CMOS circuitry.
Also included in this specification is a description of an integrated circuit ON/OFF switch. The prior art for ON/OFF switches includes the mechanical ON/OFF switch which requirs a separate switch dedicated to power switching. The advantage of an integrated ON/OFF switch is that the integrated ON/OFF switch is included in the keyboard and can also be used for other functions. Except for CMOS ON/OFF switches, prior ON/OFF switches have required a constant current flow of a significant degree thus reducing battery life of battery operated microcomputer systems. The disclosed integrated ON/OFF switch requires an insignificant amount of power while in the OFF state without CMOS fabrication.